1. Field of the Invention
The present invention relates to an image processing apparatus such as an image encoding apparatus and an image decoding apparatus suitably used in a color copying machine, a color printer, a color facsimile apparatus, a database, or the like.
2. Related Background Art
Methods of dividing an image into blocks, performing orthogonal conversion, and quantizing and encoding coefficients of the orthogonal conversion result are known as conventional methods of encoding color images, as disclosed in U.S. patent application Ser. Nos. 185,024, 261,276, 312,755, and 362,014 filed by the present assignee.
In the conventional examples, since the coefficients are quantized upon orthogonal conversion, high-frequency components are lost, and ringing occurs in edge portions, and particularly, character portions, and blurring occurs in edges of reproduced line image portions such as characters, thereby greatly degrading image quality.
To the contrary, techniques for performing encoding in accordance with whether an input image is an edge portion or not are disclosed in a new U.S. patent application based on Japanese Patent Application No. 1-333836 proposed by the present assignee. With this arrangement, the ringing phenomenon described above can be prevented to obtain a high-quality image.
Even in the above new U.S. application, there is much room for improvement in processing of a line image portion of a specific color among different line image portions.
For example, reproducibility of a black line image portion which is most frequently used in a general document and a general image, that is, reproducibility of a black character is desired to be subjected to special processing since this reproducibility greatly influences image quality.
In Japanese Patent Application Laid-Open No. 62-82723, run length encoding is performed for a "0" run. However, when a specific pattern is included in a block having a predetermined length, this pattern is block-encoded. However, no consideration is taken for the presence of an edge in an image.
When an image obtained by decoding encoded data is to be reproduced as a hard copy of four colors, i.e., Y (yellow), M (magenta), C (cyan), and K (black), a black inking signal is required to print a black line image in only black so as to clearly print the black line image. The processing sequence for the black inking signal is undesirably complicated.
In order to improve quality of a black character portion, it is possible to independently encode a character portion and other portions in accordance with different methods. When only the black character portion is extracted, a peripheral portion of a black character is included in other portions if an image is read from, e.g., an image scanner. The peripheral portion forms an edge portion, and the edge portion causes degradation of orthogonal conversion encoding efficiency.
In a small letter of the Ming type, since a space between horizontal lines in the letter is very small, the space may often be omitted when it is read from an image scanner.
Demand has arisen for providing a method of performing encoding with high efficiency without degrading quality of characters when characters of a plurality of colors are present in an original.
In extraction of a line image portion of a predetermined color of input image data and encoding of this portion independently of other portions, when only the line image portion is simply eliminated, this processing is unsatisfactory due to precision of an extracting means and the like. For example, a high-frequency portion around a color character is left uneliminated. This problem is also posed in image processing except for encoding (e.g., color conversion processing).